1. Field of the Invention
This invention relates generally to the grid structure for a color cathode ray tube, for example, the color picture tube of a color television receiver and more particularly is directed to providing an improved frame for such grid structure.
2. Description of the Prior Art
It has been proposed, for example, as disclosed in detail in U.S. Pat. No. 3,638,063, issued Jan. 25, 1972, and having a common assignee herewith, that a color cathode ray tube of the type having a screen composed of laterally successive arrays of vertical phosphor stripes emitting light of three different colors, respectively, when irradiated by three respective electron beams, for example, as in the color picture tubes of the Trinitron (Trademark) color television receivers available commercially from Sony Corporation should be provided with a grid structure comprised of a frame or support in which there is mounted a grid element defining a plurality of vertical grid wires with slits therebetween each corresponding to a respective array of the phosphor stripes and through which the three electron beams are intended to pass for irradiating or impinging upon the respective stripes. In such known grid structure, the frame is formed of a pair of elongated frame elements disposed in substantially parallel, spaced apart relation at the top and bottom, respectively, of the grid structure, and a pair of mechanically resilient brace members, preferably of C-shaped configuration, extending between the frame elements at the opposite sides of the grid structure for maintaining the frame elements in their spaced apart relation. The C-shaped brace members preferably have their ends secured to the frame elements substantially at the Bessel points of the frame elements, and, at the time when the grid wires are welded or otherwise affixed to the frame elements, the grid wires are longitudinally tensionsed and the brace members are prestressed by forces acting on the frame elements at predetermined points adjacent the end portions of the latter so as to urge the frame elements toward each other. By reason of the foregoing, after the grid wires have been affixed to the frame elements and the forces for prestressing the brace members have been removed from the frame elements, the grid wires are longitudinally tensioned in accordance with a predetermined pattern. Thus, for example, if the brace members are prestressed at the time of the welding of the grid wires to the frame elements by forces acting on the frame elements at points closer to the ends of the latter than the Bessel points or other locations at which the ends of the brace members are affixed to the frame elements, then the grid wires close to the opposite sides of the grid structure will have a greater longitudinal tension applied thereto than the grid wires in the central portion of the grid structure. The foregoing distribution of longitudinal tensions in the grid wires ensures that, when the color cathode ray or picture tube is subjected to an impact or vibration, the resulting vibration of the grid wires will have a smaller amplitude adjacent the opposite side portions of the grid structure than at the center thereof, as is known to be desirable since the electron beams travel over greater distances when impinging on the side portions of the screen than at the center of the latter.
It will be appreciated that, during operation of the described color cathode ray or picture tube, the electron beams, in scanning the screen, also irradiate the grid wires and, as a result thereof, the temperature of the grid wires is raised to approximately 100.degree. C. to 130.degree. C. so that the grid wires are subjected to thermal expansion. However, the thermal expansion of the grid wires is accompanied by a more or less corresponding expansion of the prestressed brace members with the result of the desired longitudinal tensioning of the grid wires is substantially maintained.
In order to perform in the above-described manner, the frame of the prior art grid structure has to have a high mechanical strength, particularly for withstanding the prestressing of the brace members required to maintain the desired longitudinal tension in the grid wires when the latter are subjected to substantial thermal expansion. In order to achieve such high mechanical strength, the frame of the prior art grid structure has been made of steel, such as carbon steel and the like, and has been given relatively large cross-sectional areas so as to be of relatively great weight, particularly in the case of a grid structure for use in a large cathode ray tube. In order to reduce the weight of the grid structure according to the prior art, it has been proposed to form parts of the frame, for example, the brace members thereof of hollow metal tubing which may be of circular cross section. It will be appreciated that a hollow metal tube or cylinder of circular cross section has the same section modulus about all of its axes passing through the centroid of its circular cross-section. Further, when the hollow tube or cylinder is compared with a solid member of the same cross-sectional area and weight, the hollow tube or cylinder is found to have a substantially greater maximum section modulus or, conversely, a hollow tube or cylinder of substantially smaller cross-sectional area and weight than a solid member can be provided with the same section modulus as the latter. Thus, the prior art grid structure having the brace members of its frame formed of hollow metal tubing of circular cross section can have a sufficiently high mechanical strength without being excessively heavy.
However, the hollow metal tubing for forming the brace members of the known grid structure is substantially more expensive than corresponding structural elements of solid material and such increased cost is rather large when the metal tubing has an outer diameter smaller than 20 mm so as to require numerous steps for its manufacture. Moreover, when the brace members or other structural elements of the frame included in the known grid structure are formed of hollow metal tubing, there is the danger that air remaining within such metal tubing after the envelope of the cathode ray tube has been evacuated and sealed will leak or escape into the envelope so as to disturb the vacuum required therein. Therefore, when using the prior art grid structure having its brace members formed of hollow metal tubing, it has been necessary to either carefully evacuate all of the air from the metal tubing or to completely weld closed each end of the metal tubing when welding the brace members to the frame elements. Such welding of the metal tubing forming the brace members is time consuming and costly.